Sanitation indicator

ABSTRACT

A device for monitoring thermal energy input and displaying the relationship of the thermal energy input to a selected time/temperature relationship. The device employs an indicating material which, when melted, expands and flows into a narrow channel to provide an irreversible, visible indication of the thermal energy to which the device has been exposed.

FIELD OF THE INVENTION

The present invention relates to thermal energy measurement. Morespecifically, the present invention relates to a device for monitoringthermal energy input to a sanitizing environment and is particularlyuseful for accurately monitoring thermal energy input over a short timeinterval. An inexpensive, simple, disposable device can be constructedaccording to the present invention to monitor and display a relationshipbetween thermal energy input and a selected time/temperaturerelationship, such as the thermal death time curve for a selectedmicroorganism.

PRIOR ART

A great number of industrial applications involve the application ofheat energy to a particular environment for a particular length of timein order to accomplish a desired result. Sanitation procedures for thedestruction of certain microorganisms commonly involve subjectingmaterials to be sanitized to a controlled environment wherein heat isapplied for a required length of time. The times and temperaturesemployed will vary depending on the sensitivity to temperature of thearticles to be sanitized and the resistance of the microorganism totemperature. For example, microorganisms have characteristic thermaldeath time curves which indicate the lethal temperature/timerelationship which prevail and dictate the necessary exposures.Pasteurization conditions are dictated by siuch considerations.

Various techniques have been developed to monitor and indicate thethermal energy input to a particular sterilizing environment. Althoughelectronic monitoring techniques can be employed, the equipment isexpensive and is not suited to certain sanitizing activities such assanitizing dishes and other food-related utensils in mechanicaldishwashing equipment.

Conventional thermometers are not useful for this purpose since they donot indicate a time/temperature relationship and provide no irreversiblerecord of the sanitizing activity. Sanitation indicators have beendeveloped which employ indicating compounds having specific meltingpoints. See, for example, U.S. Pat. No. 3,324,723. Other indicators havebeen developed which rely on a temperature accelerated chemical reactionto cause color change in an indicator. These devices are not useful toaccurately reflect the thermal death curve for microorganisms. Yetanother device has been described in U.S. Pat. No. 3,981,683, whichemploys an organic compound having a melting point slightly higher thanthe sterility temperature to be monitored. The device is constructed toallow steam to diffuse through the device and depress the melting pointof the organic compound in order to reflect the contribution of humidity(steam) to the sanitizing conditions.

SUMMARY OF THE INVENTION

The sanitation devices described in the prior art have not provided asatisfactory means of precisely measuring thermal energy input into aparticular sanitizing environment and indicating the relationship of thethermal energy input to the thermal death time curve for microorganisms.The present invention provides a simple, reliable device which can beplaced in a sanitizing environment and which will accurately monitor anddisplay the thermal energy input to such environment. In addition, thedevice can be constructed to accurately indicate the relationship of thethermal energy input to the thermal death time curve for selectedmicroorganisms.

The energy monitors of the present invention comprise an indicatingmaterial having a selected melting point which, when melted, expands.The expansion of this indicating material is monitored to provide anirreversible indication of the thermal energy input to which the devicehas been exposed. A display means is provided to indicate therelationship of the thermal energy input to the energy level sufficientto kill selected microorganisms. In a preferred embodiment, the devicecomprises a body member and at least one fixed-volume reservoir cavityin the body member. The device further includes a display means remotefrom the reservoir cavity. The device has a fixed-volume flow channelconnecting the reservoir cavity and the display means, the dimensions ofthe flow channel being such as to allow passage of liquid therethrough.The reservoir cavity is completely filled with a solid indicatingmaterial which is capable of changing to an expanded, liquid phase at aselected elevated temperature. Thus, the volume of the indicatingmaterial is a function of temperature. The quantity of indicatingmaterial and the expansion properties of the material must be such as tocompletely fill the reservoir cavity and flow channel when in the liquidstate. The device also includes means for relieving pressure generatedby expansion of the indicating material.

When such a device is maintained above a given temperature threshold,the solid indicating material melts and expands into said flow channel.When the device has been above the temperature threshold for therequisite period of time, the indicating material will have flowed intothe display means. Thus, the device is capable of monitoring thermalenergy input and displaying the relationship of said input to a giventime-temperature relationship, such as the thermal death curve forselected microorganisms. The device can be used to monitor theperformance of sanitation and pasteurization equipment to ensure thatsanitation cycles are effective, e.g., reach threshold killtemperatures, without being wasteful of energy by operating attemperatures greatly in excess of those required.

While the sanitation indicators of the present invention are describedherein as being useful to monitor the sanitation cycles of commercialhot water dishwashers, these indicators can also be useful in monitoringvarious other heating cycles. Typical examples of such use are themonitoring of the pasteurization cycles for beverages such as milk,wine, beer and fruit juices as well as the sterilization cycles formedical devices and even the laundering cycle for clothing and otherfabrics.

THE DRAWING

FIG. 1 is a side view of a device according to the present invention.

FIG. 2 is a section view along line 2--2 of FIG. 1.

FIG. 3 is a top view of the device shown in FIG. 1.

FIG. 4 is a top view of an indicating device according to the presentinvention having two separate reservoir cavities.

FIG. 5 is yet another embodiment of the device of the present inventionhaving multiple reservoir cavities.

FIG. 6 is a graph showing the minimum time/temperature curve specifiedfor commercial dishwashers to ensure adequate sanitation (dashed line).The solid lines show the time/temperature relationships measured byvarious sanitation indicators prepared according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the Drawing, FIG. 1, shows a side elevation of an indicatordevice 1. The device comprises body member 3 in which there is disposeda recessed cavity 5 connected via a recessed flow channel 7 to displaymeans 9. Cover means 13 is attached to body member 3 and coversreservoir cavity 5 and flow channel 7 such that said cavities have afixed volume. Indicating material 11 is shown completely filling cavity5. Sorbent pad 9 functions as a display means and is accessible byindicating material 11 through flow channel 7. Pressure relief passage15 communicates with flow channel 7 and the atmosphere.

Body member 3 can be made of a wide variety of materials. Materials inwhich the necessary recesses and cavities can be drilled, milled,etched, cut, or otherwise formed, such as by molding techniques, can beusefully employed in the present invention. The materials may be organicor inorganic. Metals, such as aluminum, copper, etc. can be used,however, plastic materials, such as organic polymeric materials aregenerally preferred. The materials should be transparent or otherwiseprovided with windows or other means so that movement of the indicatingmaterial into the display means can be observed. In addition, thematerial should be able to retain the necessary physical properties,e.g., dimensional stability, at temperatures which would be expected tobe encountered during storage and use. Polycarbonate, cellulose acetatebutyrate and other transparent polymeric materials are particularlypreferred for use as body members in the present invention.

The channels and other cavities can be made to precisely controlleddimensions by normal injection molding techniques when the preferredplastic materials are employed.

The covering material, 13, can be a thin, flexible material which can beadhered to body member 3. Cover material 13 should be strong enough sothat it is not deformed when the indicator material 11 melts andexpands. However, cover material 13 must be sufficiently formable sothat it can be pressed into intimate contact with indicating material 11at the time of manufacturing the device as shall be described in greaterdetail hereinafter. The material employed as the cover material 13should be thermally conductive in order to rapidly transmit thermalenergy to the indicating material 11 from the surrounding environmentand to prevent solidification of the indicating material 11 as itproceeds toward sorbent pad 9. Thus, the cover material should functionas a "thermal window" responsively coupling the indicating material 11to the surrounding environment. A particularly preferred material isaluminum foil, preferably a foil having dead-soft temper. Such foils canbe provided with an adhesive backing resistant to water, heat and thelike, so that they can be adhered to, and remain in intimate contactwith, body member 3.

Sorbent pad 9 can be any material which will readily sorb indicatingmaterial 11. A paper pad is suitable. However, it has been found thatwhen the sanitation indicator is to be used in a wet environment it ispreferable that the sorbent pad 9 be fully oleophilic. As such, it willreadily sorb indicator material 11, but will resist absorption of anywater which may enter the indicator through relief passage 15. Aparticularly useful material is available commercially under the tradename "TYVEK" from E. I. Du Pont De Nemours, Inc.

Indicating material 11 can be a material which will melt at a selectedelevated temperature (i.e., a temperature above room temperature) andwhich, when melted, will expand a sufficient amount to fill cavity 5,flow channel 7 and contact sorbent pad 9.

Materials having the requisite melting and expansion properties can beprepared from readily available materials which are known in the art.For example, U.S. Pat. Nos. 3,631,721 and 4,170,956 disclose organicmaterials which can be formulated to melt at selected temperature rangesand which expand when melted. Exemplary of compositions useful in thepresent invention is a mixture of, by weight, 96 parts N-octylmyristamide, 4 parts N-octyl lauramide and 0.2 parts Spirit Soluble Fast3 B Dye (BASF Wyandotte Corp.). This composition has a melting point ofabout 66°-67° C., a specific heat of 0.58 calories per centigrade degreeper gram at 90° C. and a coefficient of thermal expansion on changingfrom solid to liquid of 0.0759. A second representative compoundcomprises, by weight, 77.5 parts N-dodecyl lauramide, 22.5 parts N-octyllauramide and 0.3 part Spirit Soluble Fast HFL Dye (BASF WyandotteCorp.). This composition has a melting point of 69°-70° C., a specificheat of 0.59 calories per centigrade degree per gram at 90° C. Acomposition comprising, by weight, 76 parts N-dodecyl lauramide, 24parts N-Octyl lauramide and 0.3 part Spirit Soluble Fast HFL Dye has amelting point of about 69° C. The time/temperature properties ofindicators using the above three compositions are shown in FIG. 6 asCurves A, B and C, respectively. The melting points noted above weredetermined using a capillary melting point apparatus heating at the rateof 1° C. per 30 seconds.

Dye is generally added to the above compositions to make them morevisible in the sorbent display pad.

FIGS. 4 and 5 of the Drawing show alternate embodiments of the presentinvention which have the capability of containing 2 or more differentindicating materials. Such devices are capable of displaying therelationship of thermal energy input in a particular environment tomultiple time-temperature relationships. Sanitation indicator 17comprises body member 19 including a first cavity 21, a second cavity 23and first and second flow channels 25 and 27, respectively. Sorbent pad29 functions as a display element and pressure relief passage 31 ventsflow channels 25 and 27 to the atmosphere.

Cavities 21 and 23 can be filled with different indicating materials(not shown) having different melting points. For example, reservoircavity 21 could be filled with an indicating material having a meltingpoint which is lower than the melting of the second indicating materialin cavity 23. Since flow channel 25 is longer than flow channel 27, thedevice will monitor and display the effect of a first temperature for agiven period of time and a second, higher, temperature for a relativelyshorter period of time.

FIG. 5 shows a sanitation indicator 33 comprising a circular body member35 which includes multiple reservoir cavities 37, 39, 41 and 43. Thesecavities are connected to sorbent pad 53 by flow channels 45, 47, 49 and51 having various lengths. Pressure relief passage 55 vents the flowchannels to the atmosphere. Indicating materials (not shown) having thesame or different melting points could be provided in each of thevarious reservoir cavities to provide a device capable of monitoring andindicating various heat histories.

The devices of the present invention find particular utility inmonitoring hot water sanitation conditions, such as a dishwashing cycle.Heating cycles for appliances such as dishwashers can be established bydetermining the time/temperature conditions required to destroy selectedmicroorganisms which are found in such environments.

The concept of "cumulative heat factor" has been accepted in the publichealth field as related to Mycobacterium tuberculosis in thepasteurization of milk. Studies using Mycobacterium phlei in water havenow established that a similar relationship exists with regard tosanitization conditions in dishwashers. Further, the cumulative effectsof time and temperature have been studied and the heat unit equivalent(HUE) required to sanitize dishes, etc., has been firmly established.

The concept of heat unit equivalents is based on the finding that 1800seconds at 143° F. (62° C.) is suficient to kill Mycobacteriumtuberculosis in milk or Mycrobacterium phlei in water. 1 second at 143°F. (62° C.) provides 1 HUE. Similarly, it has been found that only 15seconds at 161° F. (71° C.) will kill the microorganism. Thus, onesecond at 161° F. (71° C.) provides 1800/15=120 HUE's. In like manner,the HUE's for each temperature can be established. To determine thetotal HUE's produced in a heat cycle the average temperature for eachsecond is determined and the HUE's calculated for that second. The sumof the HUE's can be determined for the total cycle.

A further discussion of pasteurization, sterilization and conceptsrelating to thermobacteriology in food processing can be found in Block,Seymour S., "Disinfection, Sterilization and Preservation," 2nd Edition,Lea and Febiger, Philadelphia (1977), and in Stumbo, C. R.,"Thermobacteriology in Food Processing," 2nd Edition, Academic Press,New York (1973).

A cycle producing 3600 HUE's has been adopted by the National SanitationFoundation as the minimum sanitation requirement for commercial hotwater dishwashers. This provides a safety factor of two. Such an HUEcurve is shown as a dashed line in FIG. 6 of the Drawing.

Sanitation devices, such as institutional dishwashing machines, must becapable of cleaning the dishes and providing, as a minimum sanitizationcycle, thermal energy input corresponding to some point on the curveshown in FIG. 6. Exceeding this thermal input insures thatmicroorganisms which may be present on the dishes will be destroyed.However, it is not desirable to exceed this thermal energy input by asubstantial amount, since to do so merely wastes energy withoutproviding any beneficial effect.

The devices of the present invention can be inexpensively constructed tofaithfully monitor the thermal energy input for a dishwashing cycle anddisplay the relationship of the input to the thermal death curve shownin FIG. 6. This allows the use of minimum amounts of energy while stillproviding an appropriate safety factor. This is accomplished bybalancing the thermal capacity and heat transfer characteristics of thedevice as well as the melting point and expansion coefficient of theindicating material. In practice, such a balance is favorably struck byemploying a body member of polycarbonate and an aluminum foil cover. Thethermal conductivity of the polycarbonate is about 0.11 btu/hr ft²°F./ft whereas thwe thermal conductivity for aluminum is in excess of100 btu/hr ft² °F./ft. Thus, the aluminum foil cover acts as a thermalwindow transferring heat directly and quickly to the indicatingmaterial. This provides a device which is responsive to temperaturecycles of relatively short duration e.g. 5 minutes or less.

The fine tuning of the device is accomplished by blending variousorganic materials having the desired melting points and expansioncoefficients to closely approximate the selected thermal curve.

The present invention can be further illustrated by reference to thefollowing example.

EXAMPLE 1

A sanitation indicator according to the present invention can beprepared as follows:

A plastic body member was injection molded from polycarbonate to have astructure similar to that shown in FIGS. 1-3 of the Drawing. The nominallength of the body member was about 9 cms while the nominal width andthickness of the body member were 1 cm and 0.25 cms respectively. Thechannel 7 connecting the reservoir cavity 5 and the display means 9 was0.025 inches (0.521 mm) deep, 0.006 inches (0.52 mm) wide and 2.34inches (59.4 mm) long. It has been found that the channel should not bemade much wider than 0.15 mm so that the foil cover will not be pressedinto and plug the channel.

An indicating material was prepared by combining 96 parts by weightn-octyl myristamide and 4 parts by weight n-octyl lauramide. A dye (BASFSpirit Soluble Fast 3 B) was added in the amount of 0.2 parts by weight.This composition had a melting point of about 66°-67° C. The mixture wasmelted at 110° C. and mixed thoroughly for one hour. After separatingany particulate residue, the reservoir cavity in the body member wasfilled with about 0.05 gram of indicating material using a preheated eyedropper. Due to the surface tension of the indicating material thereservoir cavity can be filled to provide a convex-shaped charge withoutcausing the indicating material to run out of the cavity.

The indicating material was allowed to cool and solidify and was thenbuffed flush with the surface of the plastic body member by rubbing witha sheet of smooth bond paper. Following buffing, the surface of theindicator material was examined and any visible voids were filled withadditional melted indicator material and the surface rebuffed. Followingbuffing, the flow channel and other cavities in the body member werecleaned using a blast of compressed air.

A display disc comprising a sorbent pad (Tyvek, DuPont) was adhered inthe cavity at the other end of the flow channel with a polyvinylacetateadhesive. The display disc was located so as to be directly adjacent theflow channel outlet.

The reservoir cavity, flow channel, display pad cavity, and pressurerelief passage were then covered by applying an aluminum foil tapehaving a pressure-sensitive adhesive backing (3M tape #425) to the backof the plastic body member. The tape was applied with slight pressureand then trimmed to size with a razor blade. Care must be taken to avoidplugging the pressure relief passage. A body member was then placed intoa support device and the aluminum foil tape firmly pressed down using asmooth metal plate in a platen press at a press pressure of 5600 psi(3.9×10⁷ Pascals). Following this first pressing, the foil tape wasagain pressed at 1280 psi (8.8×10⁶ Pascals) using a pressing platehaving an elastomeric projecting portion corresponding in shape to thereservoir cavity. The elastomeric portion was about 0.075 inches (1.9mm) thick and was recessed into the metal plate so only about 15 mils(0.38 mm) protruded above the metal plate. The elastomeric material hada Shore A Durometer hardness of about 76. The particular elastomericmaterial chosen was a Fluorel rubber. When the pressed plate was presseddown over the body member, the elastomeric portion caused the aluminumfoil tape to be intimately pressed into contact with the solidifiedindicating material in the reservoir cavity.

Following pressing, the entire indicator was exposed to a 63°-64° C.environment for about 8 hours to condition the adhesive backing on thetape, i.e., to convert any unstable adhesive to a more stable form whichwill not react with or otherwise affect the indicating material.

The indicator prepared as described above exhibited a run-out time (i.e.the time required for the first color to appear on the display pad) of55-70 seconds in a stirred water bath at 70 plus or minus 0.1° C. withwater flowing past the indicator at a rate of 0.65 feet/sec. (19.8cm/sec). The time/temperature characteristics of this indicator areshown as curve "A" in FIG. 6. This curve closely approximates thesanitation curve (dashed line) which is acceptable as the standard.Thus, this device can be used to monitor the sanitation cycle in a dishwashing machine. For example, the device is placed on a dish in amachine and the machine run through the sanitation cycle. If therequired amounts of thermal energy has reached the dish the displayportion of the indicator will register this condition. If insufficientheat has been applied the indicator material will not have reached thedisplay area. The machine can then be adjusted to provide the necessarythermal input.

For short time, high temperature cycles indicating materials comprising77.5 parts n-dodecyl lauramide and 22.5 parts n-octyl lauramide or 76parts n-dodecyl lauramide and 24 parts n-octyl lauramide may be used tomore closely approximate the lower portion of the dashed curve in FIG.6. These compositions provide indicators having the time/temperaturecharacteristics shown in curves B and C, respectively, of FIG. 6.

What is claimed is:
 1. A device for monitoring and irreversiblydisplaying short term thermal energy input as a function of a selectedtime-temperature relationship, said device comprising, incombination:(a) a dimensionally stable body member, (b) at leastone-fixed volume reservoir cavity, in said body means, (c) a displaymeans comprising a liquid-sorbent pad located remote from said reservoircavity, (d) a fixed-volume flow channel connecting each of saidreservoir cavities and said display means, the dimensions of said flowchannel being such as to allow passage of said liquid therethrough, (e)solid indicating material completely filling each of said reservoircavities, said solid indicating material capable of changing to anexpanded liquid phase at selected elevated temperatures, the volume ofsaid indicating material being a function of temperature, the quantityof said indicating material in each flow cavity being sufficient tocompletely fill the reservoir cavity and connecting flow channel when inthe liquid state, (f) means for relieving pressure generated byexpansion of the indicating material, and (g) means covering saidreservoir, flow channel and display means for rapidly conducting thermalenergy from the surrounding environment to the indicatingmaterial,whereby when said device is exposed to said selected elevatedtemperature said solid indicating material melts, expands into said flowchannel and into said display means in response to and as a function ofthe thermal energy input.
 2. A device according to claim 1 wherein saidindicating material is an organic composition which is solid at roomtemperature.
 3. A device according to claim 1 wherein the indicatingmaterial has thermal properties such that the device displays atime-temperature relationship corresponding to a thermal death timecurve for a microorganism.
 4. A device according to claim 3 wherein saidmicroorganism is Mycobacterium tuberculosis or Mycobacterium phlei.
 5. Adevice according to claim 4 wherein said indicating material is anorganic composition comprising one or more organic compounds selectedfrom the group consisting of n-octyl myristamide and n-octyl lauramide.6. A device according to claim 5 wherein said indicating materialcomprises a dye soluble in said indicating material.
 7. A deviceaccording to claim 1 wherein said body member comprises a transparentpolymeric material.
 8. A device according to claim 7 wherein saidpolymeric material is selected from the group consisting of celluloseacetate butyrate and polycarbonate polymers.
 9. A device according toclaim 1 wherein said liquid sorbent material is an oleophilic pad.
 10. Adevice for detecting and displaying thermal energy input and displayingthe relationship of said input to a preselected time-temperaturerelationship, said device comprising in combination:(a) a thin,elongated, transparent plastic body member, having first and secondends, and first and second sides, said first side being substantiallyplanar, (b) a first recess in said first side proximate said first endof said body member, (c) a second recess in said first side of said bodymember and remote from said first recess, (d) a flow channel in saidfirst side of said body member connecting said first and secondrecesses, (e) said first recess completely filled with a solidindicating material which is capable of changing to an expanded liquidphase at elevated temperature, the volume of said indicating materialbeing a function of temperature, said indicating material containing acolored dye. (f) a pad in said second recess capable of sorbing saidcolored indicating liquid, said pad being in communication with saidflow channel, (g) a pressure relief channel in said first side of saidbody member connecting said second recess to the edge of said bodymember, and (h) a thermally conductive cover means attached to the firstside of said body member and covering said first recess to form acompletely filled, fixed-volume reservoir cavity and covering said flowchannel to form a fixed-volume flow channel, said cover means furthercovering said second recess and said pressure relief channel,wherebywhen said device is exposed to said elevated temperature said solidindicating material melts, expands into said flow channel and into saiddisplay means in response to and as a function of the thermal energyinput.
 11. A device according to claim 10 wherein the indicatingmaterial has thermal properties such that the device displays atime-temperature relationship corresponding to a thermal death curve fora microorganism.
 12. A device according to claim 11 wherein saidmicroorganism is Mycobacterium tuberculosis or Mycobacterium phlei. 13.A device according to claim 12 wherein said indicating materialcomprises one or more compounds selected from the group consisting ofn-octyl myristamide and n-octyl lauramide.
 14. A device according toclaim 10 wherein said cover means is a metal foil.
 15. A deviceaccording to claim 14 wherein said metal foil is dead soft aluminumfoil.
 16. A device according to claim 10 additionally comprising a thirdrecess in said first side of said body member and remote from said firstrecess, a flow channel in said first side of said body member connectingsaid second and third recesses, said third recess completely filled witha second, solid indicating material which is capable of changing to anexpanded liquid phase at an elevated temperature different from saidfirst solid indicating material in said first recess, the volume of saidsecond indicating material being a function of temperature, said secondindicating material containing a colored dye and wherein said covermeans covers said third recess and said connecting flow channel.
 17. Adevice according to claim 16 wherein said dye in said second indicatingmaterial is a different color from the dye in said first indicatingmaterial.